Much will depend on whether it's day or dark, and the sea state. A controlled, ie powered, smooth water ditching probably survivable in almost any civil type. Get something with a high wing loading, combine with rough sea, and maybe no power to extend flaps and run landing lights in the dark, and I reckon it would be all over for most occupants due to the airframe breaking up before it came to rest.
Even a docile BN2 Islander can hurt when it hits water at a mere 35 knots, as I know one ferry pilot who had the nose cave in enough to crush his legs when he ditched. And this guy was a past master of ditching, having done it three times before.